This invention relates to color display devices, for use in television displays, computer monitors, and the like.
Conventional displays, using cathode ray tube (CRT) display devices, operate with a series of horizontal lines written continuously on the display in a vertical progression. The lines may be written continuously from top to the bottom (progressive, continuous or non-interlaced scan) or, more traditionally in broadcast receivers the odd numbered lines may be written first on one field and the even numbered lines then written on the next field (interlaced scan). For present purposes there is no material difference between progressive and interlaced scan; they both scan line-by-line and are supplied with an essentially continuous video signal which represents what may be regarded as picture elements or pixels along successive lines. Although referred to as pixels or picture elements, in the received analog video signal the pixels are not discrete, but rather the signal is completely continuous during each line. The lines are sufficient in number to be invisible to the normal user at the normal viewing distance.
To provide color on the display a cathode ray tube has three guns which receive analog red, green and blue (RGB) color-component signals respectively, and which are arranged to place red, green and blue spots closely together on the display screen. The three elemental color areas are not superposed, but are placed side-by-side. In a traditional shadow mask tube, illustrated in FIGS. 1A, 1B and 1C of the drawings, they are in a triangular arrangement of dots. In another type of tube known as the striped tube, illustrated in FIGS. 2A, 2B and 2C, the color regions are in narrow vertical stripes down the screen. In either event, the three color components of such a triplet are derived from the same instant of the video signal, but are positioned on the display at very slightly different locations. The video signal can be said to be sampled in this process. The three points of color can be referred to as sub-pixels. The sub-pixels are sufficiently close to render the sampling invisible.
A new type of display, which may be termed a discrete color display or matrix display, is now being developed, which consists of a two-dimensional array of separate display elements. An example of such a display is a plasma display. In this case also, the sub-pixels are not coincident on the display but appear at difference places on the overall display. They may be arranged in stripes as in the striped display. In this case the separate display elements are separately addressed, each with a separate pixel value, with successive samples, which have been taken from an appropriate continuous video signal.
We have appreciated that the spatial separation of the dots or stripes on the screen means that the information is not being displayed at precisely the correct point. Putting it another way, the information that should be displayed at the red sub-pixel should differ as regards the precise instant in the red video signal from which it is taken such as to reflect the spatial difference in the location of the red sub-pixel from the green and blue sub-pixels.
We have appreciated that with existing analog CRT displays this effect is obtained by adjusting the relative timing between the instants that the red, green and blue beams excite the phosphors as they sweep across the sub-pixels. This adjustment takes place during the convergence operation, which is part of the CRT setting-up procedure and can be explained as follows. A visual convergence adjustment takes place in which the scans are laterally adjusted so as to produce the optimum image as judged by a visual observer. FIG. 3 shows the relative disposition of the sub-pixels, guns and shadow-mask apertures with superposed beam positions corresponding to three adjacent sets of sub-pixels. It will be appreciated that,for the purposes of illustration the distance from the guns to the shadow-mask has been considerably reduced compared to the distance from the shadow-mask to the pixels. FIGS. 4A, 4B and 4C show the beam positions at three successive instants of time as the beams sweep from left to right. It can be seen that the spacing;of the beams is such that two of the beams are blocked when the third is exciting the appropriate phosphor. This has the effect of three-fold interleaving the times in the red, green and blue signals when the appropriate phosphors are excited.
U.S. Pat. No. 5,604,513 describes video display apparatus using a matrix display. Analog color component signals are received and serially sampled for application sequentially to the matrix display. The inventor specifically wishes the three horizontally-spaced color components to represent the same point in time, and therefore includes a one-third pixel delay in one component signal path and a two-thirds pixel delay in another of the component paths. This in fact introduces a problem similar to that noted above.
The present invention is defined in the independent claims below, to which reference should now be made. Advantageous features are set forth in the appendant claims.
In accordance with this invention we have appreciated that a discrete digitally-driven display can, however, be improved by modifying the signal samples to take account of the sub-pixel shift.
Where the sample rate of the RGB signals is the same as that of the display, this modification preferably takes the form of an interpolation of the R and B samples to new samples which are offset by the R and B sub-pixel shift from the G samples. Where the sample rate (samples/line) of the RGB signals is different from that of the display, all three sets of samples need to be interpolated, again allowing for the shift offset.
The improvement is, in principle, applicable to both one-dimensional and two-dimensional sampling situations. One-dimensional sampling arises with a display where the colors are arranged in stripes where the sub-pixels of a color are vertically aligned, whereas two-dimensional sampling arises in particular where the sub-pixels are arranged in a group of dots. For ease of explanation the following description by way of example will be made in relation to a one-dimensional arrangement. Those skilled in the art will be able to expand the treatment to the two-dimensional situation.